I've got the Sprawl Loosefit rashguard, and it totally rocks, it has so much stitching it will never rip, and has alot of elastic down the bottom so it holds tight and doesnt roll up you when you are on the mats.

I am using a Under Armour HeatGear sleeveless T these days, tight fight and keeps me cool. I have an Atama rashguard that I feel is a little too thick, almost like a wetsuit. I have a Full Contact Fighter one which is also very comfortable and has lasted me several years now.

XFGear. There is nothing on the market that really blends the best of strength, durability, style, customization and comfort. It's one of those products that if you use it once, you will never stop using it. Plus you can design your own stuff! Can't beat it.